Ovens having lighting devices are known of many different shapes, partly ovens in which only microwave energy is used, partly so called combi-ovens in which hot air as well as specific grill elements are used. The expression microwave oven in the following is intended to cover ovens in general in which microwaves are used.
In a microwave oven the lighting device is usually provided in the roof of the cavity or in any one of its side walls. As a light source a kind of lamp or light bulb is arranged in a fitting in the cavity wall.
Lighting devices of this type must comply with a number of requirements. It must provide an even lighting of the complete cavity of desirable light intensity. Additionally lamp fitting in the cavity wall must not permit more than negligible leakage of microwaves from the cavity. Under all circumstances said leakage must be well below the limits set up by authorities in different countries.
Leakage should also be minimized since leaking microwaves may also disturb the control electronics of the microwave oven. The lamp fitting and the lamp itself may only influence the microwave field distribution in the cavity to a negligible degree, because an even field distribution is of prime importance for satisfactory cooking results. A sufficient life time of the lamp must be guaranteed, which demands a specific shape of the filament in order to minimize its antenna function with respect to microwaves and thereby its possibility of being excited by the same, or, alternatively, the lamp should be protectively positioned so that the filament is reached only by a small amount of the microwaves. The heat radiation of the lamp must be limited to avoid disturbing surrounding components, which may be a problem if, for example, the lamp is arranged behind a protecting perforated plate, requiring a more powerful light source and consequently a higher electric power and heat radiation. The lamp and the lamp fitting should occupy a space which is as small as possible both inside and outside the cavity. In general the lighting device should be suitable for production at low cost, among other things by requiring the least possible change of the cavity wall and the use of a standard type lamp.
In so called combi-ovens with a grill element the temperatures in the vicinity of the grill elements may amount to 300.degree.-400.degree. C. The lighting device must then withstand such temperature levels.
Prior art lighting devices for microwave oven comply with some of the listed requirements, some of them to a certain degree, others not at all. For example:
DE-A-36 41 420 discloses a lighting device in which the lamp is positioned in a hole in the cavity wall by means of a metal carrier provided on the outside of the cavity. In order to minimize the microwave leak through said hole and carrier a specific sleeve of metal net is provided around the lamp. In comparison with the requirements discussed above, it may be concluded among other things that the lighting device is of a relatively complicated construction and that the associated metal carrier demands considerable space outside the cavity.
DE-A-36 43 307 discloses a lighting device in which the lamp is surrounded by a sleeve of a paramagnetic material. The lamp is arranged in a so called lamp housing provided on the outside of the cavity wall. Inwardly, the cavity, the lamp, and lamp housing is covered by a protective glass. The protective glass and its fitting into the cavity wall generate microwave field disturbances and some microwave leakage through the lamp housing. The purpose of said paramagnetic sleeve is to minimize said disturbances and leakage. Similar to the construction described above relative to DEA-3641420, the lighting device of this patent is also complicated and space demanding.
DE-A-36 43 364 which corresponds to U.S. Pat. No. 4,814,952 discloses a lighting device of which the lamp is carried by a lamp fitting arranged on the inside of the cavity wall, said fitting being in some embodiments sunk into a recess in the cavity wall. The lamp protrudes as a hole into the cavity and is covered by a protective glass. The protruding position of the lamp means that the filament as well as the supporting legs thereof must be dimensioned in relation to the wavelength of the microwaves in order not to start glowing under influence from the microwaves. Accordingly, this construction puts specific demands on the shaping of the lamp. The position of the lamp in the cavity in combination with the recessed positioning of its socket in the cavity wall will influence the field distribution in the cavity.
EP-B-120 536 discloses a lighting device in which the lamp is arranged in a so called wave guide sleeve at a hole in the cavity wall. The sleeve as well as the position of the filament in the same are so selected that microwave leakage from the cavity through the hole in the cavity wall is minimized to an acceptable level. As a whole this construction is relatively complicated and space demanding. A similar construction is disclosed in U.S. Pat. No. 2 526 226 including a tube having a cut-off wavelength which is below the wavelength of the microwaves. This construction also is relatively complicated and space demanding.
DE-A-3 703 937 discloses a further construction including a cage mounted in the cavity wall for accommodating the lamp. This disclosure specifically underlines the problem to safeguard a good metallic contact between the cage and the cavity wall, and thereby also demonstrates the drawbacks involved with the use of sealing means like said cages, tubes and sleeves.